I'm setting up a fiction website using Organize Series to keep multi-chapter stories together. In some cases, though, the multi-chapter stories are themselves part of a story series. It would be like:

Series: The Lord of the Rings
Prologue: Concerning Hobbits (single post)
Maps (single post)
The Fellowship of the Ring, which is also a series:
Chapter 1: A Long-Expected Party
Chapter 2: The Shadow of the Past
...
The Two Towers, which is also a series:
Chapter 1: The Departure of Boromir
etc.

How can I make a "series" like The Fellowship of the Ring itself part of a series, like LOTR?

Using your example above; currently you cannot order the series within, "The Lord of the Rings" group in the same manner as you order posts within a series. However, I am working on an update that will attach the series creation date to all series so you can order by the date a series is created. No ETA on delivery but it will be (hopefully) within a month.

Your purchase of the Organize Series Grouping will get you a whole year of updates and I have a lot planned for improving its functionality.

Results on development system, for a "series" with a single post in it:

inside wp_postlist_count Point A Point B inside wp_postlist_count Point A Point B inside wp_postlist_count Point A Point B
This entry is part 1 of 1 in the series

On production system:

inside wp_postlist_count Point A Point B inside wp_postlist_count Point A Point B inside wp_postlist_count Point A Point B inside wp_postlist_count Point A Point B
This entry is part 1 of 2 in the series

Clearly, on the production system the function is going through an extra loop -- but I don't know why, or how to make it stop.